As shown in FIG. 7, for example, an existing industrial robot comprises a mount base 1, a turning base 2 which turns around the S axis over the mount base 1, and a robot arm 3 which is mounted on the turning base 2 and includes a plurality of joints. A stopper 4 is attached to the mount base 1 and the turning base 2 for the purpose of mechanically hindering the rotation of the turning base 2 when the operating angle of the turning base 2 reaches a preset limit and generating a signal upon detection of an overrun. As shown in FIG. 8, the stopper 4 comprises a fixed section 41 attached to the mount base 1; a movable section 42 attached to the turning base 2; a dog 43 which is attached to the movable section 42 so as to be movable in the tangential direction of a clearance circle of the turning base 2; springs 44, each of which holds the dog 43 at the center of the movable section 42; and a detector 45 for detecting the operation of the dog 43. When the turning base 2 exceeds a predetermined operating angle, the fixed section 41 comes into collision with the dog 43 and moves that dog 43. The detector 45 then detects the movement of the dog 43 and generates a signal, thereby deactivating a drive motor (not shown) of the turning base 2. If the turning base 2 turns further, the movable section 42 comes into collision with the fixed section 41, thereby mechanically hindering the turning action of the turning base 2 (as disclosed in, e.g., Japanese Patent Laid-open No. Hei. 2-160497).
The problem of the foregoing existing technique is that the limit of operating angle of the turning base cannot be set to a value in the vicinity of 360.degree. or beyond a value of 360.degree.. More specifically, the existing technique has the following problem. As shown in FIG. 8, there are two limits to the extent to which the turning base 2 turns, namely, one limit (position A) where the movable section 42 of the stopper 4 comes into collision with one side surface of the fixed section 41 as a result of counterclockwise turning of the turning base 2, and a limit (position B) where the movable section 42 comes into collision with the opposite side surface of the fixed section 41 as a result of the clockwise turning of the turning base 2. In order to set the limit of operating angle of the turning base 2 to a large value, the counterclockwise limit (position A) and the clockwise limit (position B) must be brought closer together. Further, in order to set the limit of operating angle of the turning base to a value of 360.degree. or more, the counterclockwise extent of operation of the turning base and the clockwise extent of operation of the turning base must be superimposed on each other. However, according to the existing technique, the counterclockwise limit (i.e., position A) and the clockwise limit (i.e., position B) cannot be brought closer together across the limits of size of the fixed section 41 and the movable section 42; that is, a dead space shown in the drawing. For this reason, the operating angle of the turning base 2 cannot be set to a value of 360.degree. or more. Further, even if the operating angle is set to a value less than 360.degree., it is difficult to set the operating angle to a value close to 360.degree..
As a matter of course, elimination of the stopper 4 can also be selected. However, in order to protect a drive cable or the like (not shown) of the robot arm 3 connected between the mount base 1 and the turning base 2, the operating angle of the turning base 2 must be mechanically limited. Therefore, there is a demand for a stopper capable of setting the limit of operating angle of the turning base 2 to a value of 360.degree. or more.